1. Field of the Invention
The present invention relates to a method and apparatus for treating petroleum oils e.g., crudes and residues, with antifoulants to reduce fouling of heated metal surfaces such as heat exchangers. In one aspect, it relates to a method and apparatus for monitoring fouling tendency of hydrocarbon oil stream at frequent time intervals and to adjust antifoulant treating rates in response thereto.
2. Related Art
Different crude oils have different precipitating and fouling characteristics with regard to heated metal surfaces in refinery equipment. The problem of predicting the offending substances in a particular crude oil which foul heat exchanger equipment in oil refineries and petrochemical plants has been virtually unresolved. Fouling of hydrocarbon streams, consisting of carbonaceous deposits on heat exchanger surfaces, leads to a blockage of flow and a decrease in heat transfer. Both resulting conditions severely reduce efficiency in the processing of the crude oil and other liquid hydrocarbons. If it can be predicted which fractions of the crude oils are troublesome, measures can be taken in advance to prevent this fouling by either removing the offending substances causing the deleterious deposits, or by adding antifouling additives to the flow stream to reduce deposit formation. Therefore, it would be most desirable to be able to predict these fouling substances.
All crude oils are composed of two major components, a low molecular weight oil fraction (aromatic and/or saturates), and a high molecular weight fraction insoluble in paraffinic solvents. This fraction is called C.sub.7 -asphaltenes. As used herein the term "asphaltenes" refers to these high molecular weight paraffinic insoluble asphaltenes. Asphaltenes are characterized by a high average molecular weight (about 1000 and up to 5,000) and very broad molecular weight distribution (up to 10,000) and high coking tendency.
Fouling in crude oil heat exchangers is a function of crude oil composition, presence of asphaltenes and inorganic materials, process pressure and the temperature of the metal surface. Although there are a number of mechanisms which contribute to crude oil fouling, tests have shown asphaltene/oil incompatibility is a major contributing factor.
There are a number of methods available for determining the rates of fouling of hydrocarbon streams. Conceptually, they are all similar in that they attempt to measure the change in heat transferred from a heated surface to a test fluid.
One approach is to use a test unit which is configured to allow measurement of the fluid temperature at the exit of the heat exchanger while the metal temperature of the heated tube is controlled, which is generally referred to as the Thermal Fouling Tester (TFT). This configuration provides for close simulation of refinery and petrochemical plant heat exchanger operations and provides for measurement of the significant effect of fouling which is indicated by the reduction of heat transfer. The test unit provides for a thermal fouling evaluation of the crude oil in an accelerated test which is designed to reproduce the fouling problem experienced in a refinery over several months. Acceleration is provided by carrying out test operating temperatures higher than those in a particular refinery unit, so that the prospective level of fouling can be produced in a reasonable period of time (usually 3-4 hours). Heat transfer data is obtained by holding the heater tube at a constant temperature, while measuring the change in the liquid outlet temperature. As fouling progresses, i.e., a carbonaceous deposit builds up on the heater tube surface, a decrease in the fluid outlet temperature results when using a constant outlet liquid temperature operation. The change in liquid outlet temperature with time provides the basic heat data required for comparative evaluation of untreated material and additive-treated material. The rate of change in outlet liquid temperature versus time shows relative fouling tendencies. The duration of this test is usually three hours or longer. In practice in the laboratory, the turnaround for a single sample in this equipment is about one day to obtain results. And one test unit will generally produce about 200 test in a year. However, refinery feeds and streams change constantly and multiple test, e.g., 10 to 50 or 60 per day may be required for full evaluation and control of the refinery operations. The TFT tests are based on the fouling tendency of the hydrocarbon oil under actual fouling conditions and are not predictive.
Because of the length of time to carry out these actual performance tests, the treatment of a stream generally is not responsive to frequent changes in fouling tendency of hydrocarbon oils. Normally, the treatment rate for a particular hydrocarbon oil is established and not changed for long periods of time. This either results in wasteful over-treatment or risky under-treatment because of changes in the fouling tendency of the hydrocarbon stream are not detected. There has not been a method or apparatus for continually monitoring at frequent time intervals the fouling tendency of a hydrocarbon oil stream flowing into the refinery and adjusting treatment rates in response thereto.